Local oxidation of silicon (LOCOS) has been the standard method of isolation for large-scale integration (LSI) and very large-scale integration (VLSI) circuits for approximately ten years. See, for example Appls, J.A., et al., Philips Research Reports Vol.25 (1970) pages 118-132 and also United Kingdom Patent No: 1208574.
However, several features of the technique have since been identified as having a detrimental effect on the final device structure. The lateral encroachment of the oxide into the active area (bird's beaking) severely limits the scalability of the device and an attempt to reduce this by decreasing the thickness of the field region will cause an increase in interconnect capacitance.
The standard LOCOS isolation technique currently employed involves recessing a silicon surface to form a mesa, and thermally oxidising the exposed silicon surface in steam. The top of the mesa is typically protected from oxidation by a nitride cap which is deposited on a thin stress relief oxide, to ensure minimal defect generation during oxidation. The topography presented by the isolation oxide can cause problems with photolithography prior to metal deposition, where multiple angle reflections from the bird's beak produces a `necking` effect in subsequent metal depositions. Furthermore, the profile of the bird's head does not provide a suitable surface for metal coverage primarily due to a re-entrancy of the oxide at the bottom of the bird's head/field oxide region. The coupled effect of a large bird's head and a re-entrant step could present severe problems for metal continuity, where cracking or `mouse-holing` may be evident.
Recently, several isolation techniques have been investigated in an attempt to minimise bird's beak encroachment and produce an almost planar surface-- See for example Kuang, Yi, C., Moll, J. L., Manoliu, J., IEEE Trans. Electron Dev. vol. Ed-29, no. 4, April 1982; Teng, C., IEEE Trans. Electron Devices vol. Ed-32, no. 2, February 1985 and Hui, J., Voorde, P. V., Moll, J., IEDM 1985 p. 392. In general, these methods involve complex processing steps and are often accompanied by a high defect density during oxidation.